> It would be great if we crop to a circle. There are compositions for which a circle would be much better than a square. Cropping to an ellipse would be good, too. Can you please add this feature? >
If you want to have your image show up within a circle, you can use a mask. A mask "crops" non-destructively. You can also use a shape and clip your image to the shape. By placing your image on a layer above a background image, it will appear to have been "cropped" in an ellipse. Your compositions won't suffer a bit from the fact that you've used a mask or clipped with a shape (or mask).
You don't want the transparent pixels surrounding your image—that would spoil the illusion. — Unless, of course, you intended to save it with transparency and put it into yet another document.